tail: improve throughput for -c +0 by splice()

[oech3]

Start supporting #11607 and #11785 .

tail's code path is very complex. So I do them with several PRs.
This PR covers

> truncate -s 9PB /tmp/huge
> target/debug/tail -c +0 /tmp/huge|pv>/dev/null
^C.4GiB 0:00:01 [39.4GiB/s]

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 287 untouched benchmarks
⏩ 68 skipped benchmarks¹

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_{Comparing oech3:tail-splice (3287c8b) with main (52e1bc1)}

Footnotes

1. 68 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩